1. Field of Invention
The present invention relates in general to a gear control device for a motor-vehicle gearbox, comprising:                a drum which is associated with a shaft of the gearbox to bring about the movement of one or more engagement sleeves arranged slidably on the shaft, the drum being mounted so as to be rotatable about a rotation axis and having one or more guide channels in which one or more pegs, each associated with a respective engagement sleeve, engage slidably in such a manner that the rotary movement of the drum about its own axis is converted into translatory movement of the peg (or pegs) and hence of the engagement sleeve (or sleeves), and        an actuation unit which is arranged to bring about the stepwise rotation of the drum about its own rotation axis between a plurality of angular positions each corresponding to predetermined positions of the engagement sleeve (or sleeves).        
In a gear control device of this type, it is necessary to ensure that, once a given gear ratio is engaged, and hence once the drum has moved to a given angular position, the position of the engagement sleeves corresponding to the engagement of that given gear ratio is kept unchanged until the next gear shift, without the risk of vibrations or other mechanical disturbances modifying the position of the engagement sleeves and consequently causing damage to the gearbox.
2. Related Art
U.S. Pat. No. 4,574,652 discloses a drum for a gear control device for a motor-vehicle gearbox, the drum having, on its cylindrical lateral surface, two guide channels in each of which at least one follower member is slidably guided, whereby the rotary movement of the drum about its rotation axis is converted into translatory movement of the follower member(s), wherein a series of positioning notches are provided on the cylindrical lateral surface of the drum and are spaced apart angularly along a circumference of the drum for snap-engagement by a stop member associated with the drum, and wherein the positioning notches are axially interposed between the two guide channels.
EP 1 286 088 discloses a drum for a gear control device for a motor-vehicle gearbox, the drum having, on its cylindrical lateral surface, two guide channels in each of which a follower member is slidably guided. A ring is mounted at an end of the drum, coaxially with the same, and has a series of positioning notches which are spaced apart angularly for snap-engagement by a stop member.
DE 1 202 665 discloses a drum for a gear control device for a motorcycle gearbox, the drum having, on its cylindrical lateral surface, two guide channels in each of which a follower member is slidably guided, as well as a series of positioning holes which are spaced apart angularly for snap-engagement by a stop member carried by one of the shift forks of the gear control device and are axially interposed between the two guide channels. The positioning holes are not arranged along a circumference of the drum, but along a line extending parallel to the path of the guide channel associated to the above-mentioned shift fork.
US2006/162487 discloses a drum for a gear control device for a motorcycle gearbox comprising two drum portions which are fixedly connected for rotation with each other and have each, on the cylindrical lateral surface thereof, at least one guide channel in which a follower member is slidably guided. A detent plate is mounted at an end of the drum, coaxially with the same, and has a series of positioning notches which are spaced apart angularly for snap-engagement by a stop member.